The light-emitting diode (LED) is a solid state semiconductor device. The structure of the LED comprises a p-type semiconductor layer, an n-type semiconductor layer, and an active layer formed between the p-type semiconductor layer and the n-type semiconductor layer. The light-emitting principle of the LED is the transformation of electrical energy to optical energy by applying an electrical current to the p-n junction to generate electrons and holes. Then, the LED emits a light when the electrons and the holes combine. Because of poor hole injection and low hole mobility, the overflow of the electron from the active layer into the p-type semiconductor layer is a significant problem in the LED. Electron overflow reduces both power and efficiency of the LED.
FIG. 1A illustrates a cross-sectional diagram of a conventional light-emitting device 1. The light-emitting device 1 comprises a p-type semiconductor layer 10, an n-type semiconductor layer 14, and an active layer 12 formed between the p-type semiconductor layer 10 and the n-type semiconductor layer 14. The active layer 12 comprises a plurality of barrier layers 12b and a plurality of well layers 12a stacking alternately. A p-side electron blocking layer 11 is formed between the active layer 12 and the p-type semiconductor layer 10. The p-side electron blocking layer 11 acts as an energy barrier layer to prevent electron overflow. FIG. 1B illustrates an energy band diagram of the light-emitting device 1. The energy band of the p-side electron blocking layer 11 is higher than that of the barrier layer 12b. 
FIG. 2A illustrates a hole concentration diagram of the light-emitting device 1. The hole concentration near the p-type semiconductor layer 10 is higher than the hole concentration near the n-type semiconductor layer 14. FIG. 2B illustrates an electron concentration diagram of the light-emitting device 1. Because electrons are relative light in weight compared with holes, the rate of the electron moving towards the p-type semiconductor layer 10 is more quickly than the rate of the hole moving towards the n-type semiconductor layer 14. The electron concentration near the p-type semiconductor layer 10 is higher than the electron concentration near the n-type semiconductor layer 14. FIG. 2C illustrates a radiative recombination rate diagram of the light-emitting device 1. Most of the electrons from the n-type semiconductor layer 14 recombine with the holes from the p-type semiconductor layer 10 at a position near the p-type semiconductor layer 10. Because the radiative recombination rate near the n-type semiconductor layer 14 is slow, the light generated by the electron-hole recombination in the active layer 12 of the light-emitting device 1 is reduced.